Field
Embodiments of the present invention generally relate to distributed power systems and, more particularly, to compensating for imbalances in a multi-phase distributed power system.
Description of the Related Art
Distributed power systems are typically comprised of a power source (that generates direct current (DC) power) coupled to a power converter, and a controller. The power source may be a photovoltaic (PV) module or PV panel array, a wind turbine or a wind turbine array, an energy storage device such as a battery, fuel cell, and the like. The power converter converts the DC power into alternating current (AC) power, which may be coupled directly to the AC power grid. The controller monitors and controls the power sources and/or power converter to ensure that the power conversion process operates as efficiently as possible.
One type of power converter is known as a microinverter. Microinverters typically convert DC power to AC power at the power source. Thus, each power source is typically coupled to a single microinverter. A plurality of AC power outputs from the plurality of microinverters may be coupled in parallel to the AC power grid. For systems using microinverters that each generate a single-phase output, multiple phases may be formed by manipulating the output of the microinverters such that each microinverter outputs power of a different phase. For example, for a three-phase AC power grid, three microinverters would output power for three different phases corresponding to the phases of the grid. Thus, for a large multi-phase installation, the total number of inverters may be a multiple of the number of phases; e.g., nine inverters for a three-phase system. Alternative embodiments include systems that have a disproportionate or different number of inverters per phase. Since the outputs of the microinverters are coupled in parallel directly to the AC power grid, all the parallel connected microinverters are simply phase synchronized to the AC power of the AC power grid. The synchronization may be achieved by periodically sampling the AC power grid and controlling the microinverter system to match the sampling.
The microinverter system producing the three-phase power conditionally outputs all phases as long as the phases are of substantially equal power. Should one of the outputted phases have unequal power with respect to the power on the other two phases, the system shuts down or otherwise operates with power/current curtailment to prevent the imbalance from being passed to the AC grid. Voltage imbalance may also be an issue as AC grid requirements have specific voltage thresholds at the point of common coupling (PCC) and electrical connection points (ECPs) between the grid and the distributed power system.
Therefore, there is a need for a cost effective method and apparatus to ensure power imbalance and voltage imbalance compensation to avoid a shutdown of a distributed generation system.